In general for a given ignition temperature the burning rate of a propellant is related to the pressure to which it is exposed in a manner which can be expressed mathematically by the expression EQU r=kp.sup.n
Where r is the burning rate, p is the pressure and k and an are constants which are characteristic of the propellant. Thus r increases exponentially with increasing p and log r increases linearly with log p, the graph of log r against log p being a line of slope n. In conventional propellant without ballistic modifier, the pressure exponent n has a value of 0.5 to 0.8 and for rocket propulsion the progressive increase in burning rate with increasing pressure presents problems in designing motors to withstand the pressures which could be developed. In order to overcome this problem NC based propellant compositions containing ballistic modifiers have been developed, the modifier being effective to modify the burning rate and pressure relationship so that over a useful working pressure range the pressure exponent n is reduced. In the region where n=o the graph of log r against log p contains a flat portion, termed a `plateau` and the burning is termed `plateau burning`. In some cases n is reduced to a negative value over a certain pressure range, such propellant burning being termed `mesa burning`. Ballistic modifiers causing a reduction of the pressure exponent are termed platonisation agents. Plateau burning propellants give reduced motor performance variability in the region of the plateau and mesa burning provides additional safety against the development of high pressure in the propellant container.
Ballistic modifiers (platonisation agents) commonly used include organic salts such as lead salicylate, lead stearate or lead .beta.-resorcylate and may also include additional metal salts such as copper salicylate, copper stearate or copper benzoate. The use of such ballistic modifiers is described for example, in U.S. Pat. Nos. 3,088,858, 3,923,564, United Kingdom Patent Specification 2121399 and Japanese Patent J55071690. For relatively fast burning propellants a favoured modifier comprises the reaction product of lead .beta.-resorcylate and basic cupric salicylate as described in U.S. Pat. Nos. 3,989,776 and 4,001,287.
The currently used ballistic modifiers are deficient in some respects including, in some cases, difficulty of incorporation into propellant compositions, poor reproducibility of plateaux characteristics from batch to batch, adverse effect on long term stability, ballistic drift on storage and ineffectiveness in high energy composition. There is therefore an acute need for improved ballistic modified propellant composition, especially for well platonised fast burning high energy compositions containing, when necessary, aluminium or high levels of energetic fillers such as a nitramine, for example RDX (cyclo 1,3,5-trimethylene 2,4,6-trinitramine).